When information of any kind is transmitted by means of digital data units in a network, the transmission media and data units are suitably defined for each other, or even standardized. An example is the ATM (Asynchronous Transfer Mode) where the ATM cell is the basic data unit. This is concretely referred to in the following for explanation purposes; the basic concepts illustrated in this manner also apply to other transmission installations.
The transmission medium (ATM) is designed so that the information from several sources is able to use the same physical transmission device simultaneously. Agreements are reached with the users about the transmission modalities in order to attain a good utilization rate or to prevent traffic jams. Parameter monitoring is required at an interface, either at a user interface or at a network interface, in order to make it possible to test the maintenance of the agreement on the side of the transmission device, and to ensure it if necessary. In the technical literature such traffic monitoring and control through the network is called "Usage/Network Parameter Control" (UPC/NPC). The parameter monitor responds to a violation of the agreement by discriminating the nonconforming data unit, usually by rejecting it or marking it for subsequent special treatment, for example subsequent elimination.
An agreement per virtual connection is negotiated during the connection phase, or it is renegotiated during a subsequent phase (so-called Connection Admission Control, CAC). At that time the parameters to be maintained are established. Probably the most important parameter is the number of data units that can be transmitted per unit of time, the so-called cell rate, respectively its complement the cell distance, which is also called intermediate arrival time. Different types of cell rates are defined: maximum, sustainable (as the upper limit for a mean) etc. The profession mostly uses the English expressions: "peak cell rate", "sustained cell rate", "mean cell rate". Other parameters are "cell delay variation" CDV, "burst tolerance" etc., where certain mutual dependencies may be present. In theory, the agreement between the network operator and the user can concern any parameters. The decisive ITU recommendations for the present status leave room for many possibilities. Only a single generic algorithm is presently defined for discriminating the data cells of a virtual connection in a cell stream, which do not correspond to the agreed parameters regarding the timely arrival of such data cells. It considers two contract parameters, the "Peak Cell Rate" or its complement the "Increment I", and the "Cell Delay Variation CDV" or the "Limit L". It is called a "Generic Cell Rate Monitoring Algorithm", GCRA, and is illustrated in two configuration forms as the "Virtual Scheduling Algorithm" and as the "Continuous-State Leaky Bucket Algorithm".
This GCRA "Generic Cell Rate Monitoring Algorithm", which is based on the increment I and the limit L, is usually used to perform a UPC/NPC in practice today. It must be able to test a cell stream for its conformity, meaning the maintenance of the contract (GCRA, I, L). The arriving traffic is considered to be GCRA, I, L-conformant (conforming to the contract), if no cells are rejected or marked during the pertinent UPC/NPC. If too many cells are lost from a connection, the question arises whether this can be attributed to a GCRA, I, L violation by the customer, to a malfunction of the implemented GCRA, or to a lack of resources in the network. In such a case it is important for legal considerations to have instruments which can clearly determine who is responsible for the deterioration of the service quality. The need exists therefore to be able to test the function of the traffic monitoring and control during operation ("in service, on-line"), namely without any special testing connection.
Such a test cannot be performed as is. The name "Continuous-State Leaky Bucket Algorithm" illustrates the problem correctly. It is derived from the analogy of the bucket which is constantly filled with liquid at alternating inflow rates, while the liquid runs out constantly according to the size of the leak as long as the bucket still contains any liquid. The GCRA has therefore the attribute of having a "memory" for events which can be considerably behind in time, until the leaky bucket is entirely empty. The arrival time of a cell which places the GCRA into this condition is called the regeneration point; in the following the condition itself is called a regeneration state. The momentary condition of the GCRA in a UPC or NPC is not tangible, and even if the regeneration points could be made determinable, a conformity test during operation could not be expected to have to wait for it, since it could be possible that none occurs before the end of the connection.